1. Field of the Invention
The present invention relates to a battery charge/discharge current detection apparatus for detecting a charge/discharge current of a battery mounted on a vehicle such as a passenger car or a truck.
2. Description of Related Art
There is known a remaining-battery-capacity indicating apparatus configured to detect a charge/discharge current of a battery on the basis of a voltage across a current detecting resistor connected between the negative terminal of the battery and the ground. For example, refer to Japanese Patent Application laid-open No. 6-176798. In this remaining-battery-capacity indicating apparatus, the remaining capacity of the battery is calculated by integrating the detected charge/discharge current.
In recent years, there is tendency that the number of electrical loads mounted on a vehicle which consume large power while the vehicle is moving (an electric power steering and a brake, for example) increases with development of vehicle electronization. Accordingly, there have been cases where a large current flows while the vehicle is moving, posing the following problems when the charge/discharge current of a battery is detected by use of a current detecting resistor.
(1) Since a vehicle body is used as the ground, the voltage of the ground varies depending on a current flowing through the vehicle body, causing a power supply voltage of the remaining-battery-capacity indicating apparatus to fluctuate to thereby lower the current detecting accuracy. In addition, if the temperature of the current detecting resistor excessively rises due to a current flowing therethrough, the current detecting accuracy is further lowered due to temperature drift of the resistance thereof.
(2) If the resistance of the current detecting resistor increases due to deterioration thereof, the voltage across its ends increases, and its temperature may excessively rise. The driver of the vehicle on which the remaining-battery-capacity indicating apparatus is mounted cannot notice such an abnormal condition until malfunction resulting from the abnormal condition occurs if a specific means for detecting such a condition is not provided. Furthermore, since it is not easy in many cases to find whether or not the malfunction occurred is due to inaccurate detection of the charge/discharge current of the battery, it is difficult to take quickly an appropriate measure, for example, to make repairs, or replacement. For example, when the current detecting resistor is blown, if no repairs or replacement are made, it becomes impossible to restart a vehicle engine by a starter, because the starter cannot be supplied with electric power from the battery after the engine is stopped.